variant-id in CMIP6 metadata

CMIP6 netCDF file metadata includes the variant-id global attribute which has the format r1i1p1f1, where the numbers are indices for particular configurations of:

  • r: realisation (i.e. ensemble member)
  • i: initialisation method
  • p: physics
  • f: forcing

This page documents the meaning of the indices used in the UK data delivered to the ESGF.


Initialisation method is as follows:

  • 1: Initial conditions taken from a prior simulation, as indicated by the branch-date attribute.
  • 2: As 1, but with an additional random perturbation applied to the initial state of the atmosphere.


The majority of our DECK and historical runs use v6.2.0 of the Input4MIPs forcing dataset. For the purposes of our models, v6.2.0 is identical to the subsequent and final CMIP6 release v6.2.1, as these two releases differ only in SO2 emissions from aircraft, which we do not use.


Forcing configuration for HadGEM3-GC3.1:

  • 1: Input4MIPs version v6.1.1; no ozone remapping
  • 2: Input4MIPs version v6.2.0; no ozone remapping
  • 3: Input4MIPs version v6.2.0; with ozone remapping

f1 is used only by piControl, 1pctCO2 and abrupt-4xCO2 simulations with HadGEM3-GC3.1-N216ORCA025, which were started with v6.1.1 before the updates in v6.2.0 were released. For this model, the only difference between these two versions is that historical stratospheric aerosol properties are updated in v6.2.0 to remove errors in some years. The update was applied in such a way as to preserve the time-mean global mean radiative forcing of the historical 1850-2014 average, which acts as the forcing dataset for pre-industrial simulations. Therefore, for the purposes of the piControl, 1pctCO2 and abrupt-4xCO2 experiments, v6.1.1 is consistent with v6.2.0.

Ozone remapping is applied to ensure that the spatial distribution of ozone is consistent with the model’s thermal tropopause, to avoid non-physical climate feedbacks; see Hardiman et al (2019) for details.


Forcing configuration for UKESM1:

  • 1: not used
  • 2: Input4MIPs version v6.2.0
  • 3: Input4MIPs version v6.2.0; historical simulations use 1850 forcing for stratospheric aerosol surface area density (SAD)

f3 is used only by three (out of 19) UKESM1 historical simulations which inadvertently employed periodic 1850 SAD data, instead of 1850-2014 timeseries. SAD is used by heterogeneous chemistry, and this forcing difference impact on the variability of stratospheric ozone, which in these three ensemble members will not experience the same decline following volcanic eruptions as in the other 16 UKESM1 historical members. Outside of the stratosphere we do not expect this difference in forcing to have a significant impact on model behaviour and we recommend that users who are not analysing stratospheric chemistry include these three members in the main historical ensemble.